The present invention relates to a CDMA (Code Division Multiple Access) radio communication technique and, more specifically, a transmission power limiting technique of limiting the transmission power of a radio wave transmitted from a radio base station apparatus to a terminal to an allowable average power or less, i.e., a transmission signal generating circuit and a radio base station communication apparatus using the circuit.
In a mobile communication system such as a cellular phone system, a radio base station communication apparatus uses a CDMA spread spectrum technique of performing radio communication between a base station and a terminal by using an information signal obtained by time-division multiplexing of signals spread by orthogonal codes.
In such a CDMA scheme, in order to suppress waveform distortion in a transmission power amplifier within a tolerance limit, the transmission power must be limited to prevent a radio wave from being transmitted from a radio base station communication apparatus to a terminal with a total transmission power exceeding a predetermined allowable average power.
A conventional transmission power limiting scheme will be described next with reference to FIG. 10. FIG. 10 shows the arrangement of a conventional CDMA radio base station communication apparatus. A CDMA radio base station communication apparatus 22 is wire-connected to a radio base station controller 21 to exchange communication data and control data with the radio base station controller 21.
Downlink processing of transmitting data from the radio base station communication apparatus 22 to a terminal will be described first. A transmission information signal input from the radio base station controller 21 is sent to a channel coding unit 16 through a transmission path terminating unit 19 to be converted into N-channel (N is an integer equal to or more than 2) transmission data to be transmitted as a CDMA signal. The data are then transferred to a transmission signal generating circuit 30.
In the transmission signal generating circuit 30, the transmission data from the channel coding unit 16 are spread by a spreading processing unit 31 using a predetermined spreading code, and the transmission power is adjusted on the basis of power information designated by the radio base station controller 21, thereby generating desired spread transmission signals.
All these N-channel spread transmission signals are added/combined by an N-input adding circuit 33. The resultant signal is output as a transmission output to a modulation circuit through a limiter 20. In this case, a total transmission power measuring circuit 34 is provided to measure the total average transmission power of the output from the N-input adding circuit 33. The measured total average transmission power information is sent to a common control unit 18 and is transmitted to the radio base station controller 21 through the transmission path terminating unit 19.
In uplink processing of transmitting data from a terminal to the radio base station communication apparatus 22, a channel decoding unit 17 performs despreading, RAKE combining, channel decoding, and the like of a reception signal from the demodulation circuit, and the obtained demodulated data is sent to the radio base station controller 21 through the transmission path terminating unit 19.
In this arrangement, the total average transmission power value obtained from the output from the N-input adding circuit 33 is always informed to the radio base station controller 21. If this value exceeds a preset allowable average power, the radio base station controller 21 limits transmission power to prevent the power value from exceeding the allowable average power by reducing the power information of transmission data to the radio base station communication apparatus 22.
In such a conventional technique, however, since the delay time is long in a control loop, high-speed control cannot be done. Although the limiter 20 limits the amplitude value of a transmission output when it exceeds an allowable average power, the aim of this operation is to limit instantaneous power but is not to limit average power.
As described above, therefore, in the conventional transmission power limiting scheme, the first problem is that since the powers of all the transmission channels are uniformly controlled in accordance with a total transmission power value, even the transmission power of a transmission channel which is preferably kept constant is controlled in the same manner as the remaining transmission channels.
The second problem is that since the power of a transmission channel is controlled by the radio base station controller 21 instead of the radio base station communication apparatus 22 itself, the control speed in transmission power limiting operation is low, and a quick change in power cannot be promptly limited.
Conventionally, there have been proposed techniques of categorizing transmission channels into a plurality of power classes and limiting the transmission powers of radio waves transmitted from a radio base station apparatus by controlling transmission powers on a power class basis (see, for example, Japanese Patent Laid-Open No. 2002-190774). According to this technique, a compression ratio is obtained for each power class, and the transmission power of each channel belonging to the corresponding power class is individually controlled. However, there has not been disclosed any specific circuit arrangement associated with transmission power limitation. If, therefore, the above technique is applied to a known radio base station communication apparatus, control is required through the radio base station communication apparatus, posing a problem similar to the second problem. In addition, since transmission power control needs to be performed for each transmission channel, control for transmission power limitation is complicated.